Yoke of stepping motor and method for manufacturing the same

ABSTRACT

In a method for manufacturing yokes (3, FIG. 1) to be used in a permanent magnet type stepping motor (10, FIG. 2), a surface-treated dead soft steel sheet such as nickel-plated steel sheet or aluminized steel sheet containing carbon in 0.01 wt. % or less is press-worked to form yoke (3) having plural pole teeth (3a, FIG. 1) on its inner periphery, and then the yoke is annealed in an inactive gas atmosphere in a temperature range of 750°-820° C. to remove residual stress, and permeability of the yoke is improved.

This is a division of application Ser. No. 07/557,991, filed July 26,1990 U.S. Pat. No. 5,020,211.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an improvement in a yoke of steppingmotor and method for manufacturing the same.

2. Description of the Prior Art

In recent years, a permanent-magnet type stepping motor (hereinafterabbreviated as PM type stepping motor) has been advanced to belight-weighted, thinned, shortened and small-sized. Furthermore, the PMtype stepping motor is required to be manufactured in a low cost and tohave high performance.

A conventional PM type stepping motor 10 is described referring to FIGS.1 and 2. FIG. 1 is a perspective view showing conventional yokes 3 and 3which are used in the conventional stepping motor 10. FIG. 2 is across-sectional side view showing the constitution of the conventionaland typical stepping motor 10.

In FIG. 1, which is an exploded perspective view of an upper stator 11a,an upper yoke 31 and a lower yoke 32, which are made of a magneticmaterial such as low carbon steel sheet, respectively have plural upwardpole teeth 31a, . . . and plural downward pole teeth 32a, . . . on theirinner periphery parts. A coil is wound on a bobbin 2 of an insulationmaterial. The upper yoke 31 and the lower yoke 32 are fitted to theupper and the lower faces 2a and 2b of the bobbin 2, respectively. Thepole teeth 31a, . . . and 32a, . . . are fit in an inner hole 2c of thebobbin 2 in a manner that the pole teeth 31a, . . . of the upper yoke 31and the pole teeth 32a, . . . of the lower yoke 32 make alternate row ofcircularly disposed magnetic pole pieces. Thus, an assembly of the upperyoke 31, the coil 1 on the bobbin 2 and the lower yoke 32 constitutesthe upper stator part 11a.

As shown in FIG. 2, the stator 11 comprises the upper stator part 11aand a lower stator part 11b. The lower part 11b is configurated quiteequivalent to the upper part 11a. The upper part 11a and the lower part11b of the stator 11 are tightly piled up. The coils 1a and 1b of theupper and lower parts 11a and 11b of the stator 11 are, for example,alternately magnetized by a known stepping-motor driving circuitry. Acup-shaped frame 4 is made of a magnetic material e.g. dead-soft steelsheet. Outer peripheries of the stator 11, namely outer periphery edgesof the yokes 31, 31 and 32, 32 contact the inner vertical face 4b of theframe 4, to form magnetic circuits. The frame 4 has a bearing 6 at itstop center part for rotatably bearing a shaft of a rotor 5. The rotor 5comprises a cylindrical core 5b made of non-magnetic die-cast orindustrial resin and a plurality of vertical rod-shaped permanentmagnets 5c, . . . provided circularly on cylindrical outer face of thecore 5b. The rotor 5 is rotated by magnetic attraction and magneticrepulsion between the pole teeth 31a, . . . and 32a, . . . and thepermanent magnets 5c, . . . . A bracket 8 having a bearing 7 forrotatably bearing lower part of the shaft 5a of the rotor 5 is fit in anopening 4a of the frame 4.

In the conventional PM type stepping motor 10 constituted as mentionedabove, the yokes 3, . . . are made of galvanized low carbon steel sheet(galvanized steel sheet) which has a good workability. And the yokes 3,. . . are formed by press working. Such a galvanized low carbon steel,however, has low permeability as a magnetic material after being pressworked. Therefore, the PM type stepping motor using the yokes 3, . . .made of such a galvanized low carbon steel sheet after press working cannot have high performance. Accordingly, for improving the performance ofthe PM type stepping motor 10, the yokes 3, . . . made of the galvanizedlow carbon steel sheet must be annealed for increasing the permeabilitythereof after forming to the shape shown in FIG. 1 by the press working.Such annealing of the yokes 3, . . . is executed in an inactive gasatmosphere at a temperature of about 800° C. for relieving internalstress caused by the press working so as to increase the permeability ofthe yokes 3, . . . .

In the above-mentioned case, zinc on the galvanized steel, however, ismelted by such a high temperature during the annealing. Therefore, thereis a problem that the annealed yoke 3 lose rust-preventive nature. Forsolving the problem, a low carbon steel which has not been treated withany surface treatment is used and the above-mentioned annealing isexecuted after forming the yokes 3, . . . by the press working. In thiscase, however, there is another problem that a rust-preventive platingprocess is necessary after the annealing. Furthermore, there is stillother problems that the yokes 3, . . . formed by the press working maybe deformed during in cargo working for carrying the yokes 3, . . . toand from the plating shop and that the deformed yokes 3, . . . have tobe corrected.

SUMMARY OF THE INVENTION

Purpose of the present invention is to solve the above-mentionedproblems and to provide an improved method for manufacturing yokes to beused in a stepping motor, thereby reducing cost for manufacturing theyoke and increasing permeability of the yoke.

Method for manufacturing a yoke to be used in stepping motor inaccordance with the present invention has steps of:

press-working a non-galvanic surface-treated dead soft steel sheet toform a yoke piece having plural pole teeth provided on its periphery;and

annealing the yoke piece in an inactive gas at an annealing temperatureof about 800° C. to relieve internal stress caused by the press-working.

The surface-treated steel sheet which is, for example, a nickel-plateddead soft steel sheet or aluminized dead soft steel sheet has a goodworkability by the press working. And the plated nickel or aluminum maynot be dissolved, peeled or flaked even when annealing is executed at ahigh temperature for removing permanent set so as to increase thepermeability of the yoke.

While the novel features of the invention are set forth particularly inthe appended claims, the invention, both as to organization and content,will be better understood and appreciated, along with other objects andfeatures thereof, from the following detailed description taken inconjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the perspective view showing external appearance of yokes tobe used in the typical permanent magnet type stepping motor.

FIG. 2 is the cross-sectional side view showing the constitution of thetypical permanent magnet (PM) type stepping motor.

FIG. 3 is a diagram showing a characteristic curve of permeability of asteel against annealing temperatures in accordance with the presentinvention.

It will be recognized that some or all of the Figures are schematicrepresentations for purposes of illustration and do not necessarilydepict the actual relative sizes or locations of the elements shown.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of method for manufacturing yokes of a permanentmagnet type stepping motor in accordance with the present invention isdescribed.

Shape of the yokes in accordance with the present invention issubstantially the same as that of the conventional one shown in FIG. 1.At first, surfaces of a steel sheet is nickel plated by knownelectroplating or the like method. Thickness of surface nickel film is,for example, about 3 μm. As a material of the steel sheet, a dead softsteel containing carbon at a content of 0.01 wt. % or less is used.After that, the yoke 3 having plurality of pole teeth 3a, . . . aroundan inner periphery thereof is formed by press working. At this time, thesurface of the yoke 3 (or steel sheet) is partially elongated about 30%by stress due to reducing working or drawing working. The plated nickel,however, does not peel off or is flaked.

Next, the yoke 3 is annealed about one hour under an inactive gasatmosphere at a temperature in a range of 750°-820° C. for removingresidual stress induced by the press working. FIG. 3 is a diagramshowing a characteristic curve of permeability of a steel sheet againstannealing temperatures in accordance with the present invention. Therange of 750°-820° C. of the annealing temperature is obtained from FIG.3. Thereby, permeability of the yoke 3 is improved. In the annealingprocess, the atmosphere need not be decarbonized, since thenickel-plated dead soft steel sheet containing very few carbon is used.Therefore, the annealing can be executed in the inactive gas atmosphere.

Hereupon, when the annealing temperature is lower than said lower limitof the predetermined temperature e.g. between 750° C. and 820° C., theresidual stress can not be fully removed. On the other hand, when theannealing temperature is higher than 820° C., the plated nickel on thesurface of the yoke 3 is peeled off or flaked. Therefore, the 750°-820°C. range of temperature during which the most effective annealing forimproving the permeability of the yoke 3 (or the steel) is executed wasfound through many experiments as shown in FIG. 3. The thickness of theplated nickel was decided so as to resist against the peeling in thepress working and anticorrosion after pressing. Even when the thicknessof the plated nickel was increased more than 5 μm, the anticorrosion wasnot improved a more. As a result, the maximum thickness of the platednickel was limited as 5 μm in view of economy.

As the material to be plated on the surface of the steel, aluminum isalso usable; since it can also be press-worked without peel off orflaking, it does not damage the magnetic characteristic of the steel bydeep diffusion into the steel sheet, and it prevents corrosion. Thealuminum-plated or aluminized steel sheet does not make rust even afterleaving for 300 hours under a condition of 40° C. and 90% of humidity.Even though the aluminized steel is discolored to be dark by mutualdiffusions between iron and aluminum in the annealing, the anticorrosionability of the aluminized steel is not damaged.

According to the manufacturing of the yoke in the above-mentionedembodiments a plating process after press-working is no more necessary,since the press-working and annealing are carried out on thealready-surface-treated heat resistant steel sheet such as nickel-platedsteel sheet or aluminized steel sheet. And there is no more possibilityof deforming the worked yoke during the transportation to and from theplating shop. As a result, a high performance PM type stepping motor isprovided.

Although the invention has been described in its preferred form with acertain degree of particularity, it is understood that the presentdisclosure of the preferred form has been changed in the details ofconstruction and the combination and arrangement of parts may beresorted to without departing from the spirit and the scope of theinvention as hereinafter claimed.

What is claimed is:
 1. Magnetic yoke of a stepping motorcomprising:ring-shaped magnetic sheet, a plurality of teeth-shapedmagnetic pole pieces provided to constitute bent-up teeth which aresubstantially vertical to the face of said ring-shaped magnetic sheet,connected integrally at the periphery of said ring-shaped magneticsheet; and corrosion protective metal layer plated on surfaces of saidring-shaped magnetic sheet and said plurality of teeth-shaped magneticpole pieces, wherein said corrosion protective metal layer is of nickel.2. Magnetic yoke of a stepping motor comprising:ring-shaped magneticsheet, a plurality of teeth-shaped magnetic pole pieces provided toconstitute bent-up teeth which are substantially vertical to the face ofsaid ring-shaped magnetic sheet, connected integrally at the peripheryof said ring-shaped magnetic sheet; and corrosion protective metal layerplated on surfaces of said ring-shaped magnetic sheet and said pluralityof teeth-shaped magnetic pole pieces, wherein said corrosion protectivemetal layer is of aluminum.
 3. Magnetic yoke in accordance with claim 1,whereinsaid magnetic sheet and said magnetic pole pieces are of deadsoft steel sheet having a carbon content of below 0.01 wt. %. 4.Magnetic yoke in accordance with claim 2, whereinsaid magnetic sheet andsaid magnetic pole pieces are of dead soft steel sheet having a carboncontent of below 0.01 wt. %.